The present disclosure generally relates to acidizing subterranean formations, and, more specifically, to methods for mitigating the generation of calcium-containing precipitates during acidizing operations.
Treatment fluids can be used in a variety of subterranean treatment operations. Such treatment operations can include, without limitation, drilling operations, stimulation operations, production operations, sand control treatments, and the like. As used herein, the terms “treat,” “treatment,” “treating,” and grammatical equivalents thereof refer to any subterranean operation that uses a fluid in conjunction with achieving a desired function and/or for a desired purpose. Use of these terms does not imply any particular action by the treatment fluid or a component thereof, unless otherwise specified herein. Illustrative treatment operations can include, for example, drilling operations, fracturing operations, gravel packing operations, acidizing operations, scale dissolution and removal operations, sand control operations, consolidation operations, and the like.
Acidizing operations may be used to stimulate a subterranean formation to increase production therefrom. During an acidizing operation, an acid-soluble material in the subterranean formation can be dissolved by one or more acids to expand existing flow pathways in the subterranean formation, to create new flow pathways in the subterranean formation, or to remove acid-soluble precipitation damage in the subterranean formation. The acid-soluble material being dissolved by the acid(s) can be part of the native formation matrix or can have been deliberately introduced into the subterranean formation in conjunction with a stimulation or like treatment operation (e.g., proppant or gravel particulates). Illustrative substances within the native formation matrix that may be dissolved by an acid include, but are not limited to, carbonates, silicates and aluminosilicates. Other substances can also be dissolved during the course of performing an acidizing operation, and the foregoing substances should not be considered to limit the scope of substances that may undergo acidization. As further discussed below, certain components dissolved during an acidizing operation can be problematic and possibly detrimental for future production from the subterranean formation.
Carbonate formations can contain minerals that comprise a carbonate anion (e.g., calcite (calcium carbonate) and dolomite (calcium magnesium carbonate)). When acidizing a carbonate formation, the acidity of the treatment fluid alone can be sufficient to solubilize the carbonate material by decomposing the carbonate anion to carbon dioxide and leeching a metal ion into the treatment fluid. Both mineral acids (e.g., hydrochloric acid) and organic acids (e.g., acetic and formic acids) can be used to treat a carbonate formation, often with similar degrees of success.
Siliceous formations can include minerals such as, for example, zeolites, clays, and feldspars. As used herein, the term “siliceous” refers to a substance having the characteristics of silica, including silicates and/or aluminosilicates. Most sandstone formations, for example, contain about 40% to about 98% sand quartz particles (i.e., silica), bonded together by various amounts of cementing materials, which may be siliceous in nature (e.g., aluminosilicates or other silicates) or non-siliceous in nature (e.g., carbonates, such as calcite). Acidizing a siliceous formation or a formation containing a siliceous material is thought to be considerably different than acidizing a carbonate formation. Specifically, the mineral and organic acids that can be effective for acidizing a carbonate formation may have little effect on a siliceous formation, since these acids do not effectively react with siliceous materials to affect their dissolution. In contrast, hydrofluoric acid, another mineral acid, can react very readily with siliceous materials to promote their dissolution. Oftentimes, a mineral acid or an organic acid can be used in conjunction with hydrofluoric acid to maintain a low pH state as the hydrofluoric acid becomes spent during dissolution of a siliceous material. The low pH state may promote initial silicon or aluminum dissolution and aid in maintaining these substances in a dissolved state. Moreover, the additional acid may also promote dissolution of non-siliceous materials in the subterranean formation as well.
Despite the advantages that can be realized by acidizing a siliceous formation, there are significant issues that can be encountered during such operations. Dissolved silicon and aluminum can sometimes react further, particularly in the presence of alkali metal ions, to produce damaging precipitates that can often be more detrimental for production than if the acidizing operation had not been performed in the first place. In addition, in subterranean formations containing both a siliceous material and a carbonate material, precipitation of calcium fluoride, a fairly insoluble salt, can also be exceedingly problematic. Moreover, calcium fluoride precipitation can decrease the quantity of fluoride ions that are available to solubilize the siliceous material. For these reasons, conventional acidizing operations have often been difficult to conduct in siliceous formations containing more than about 5% carbonate minerals.
The equilibrium solubility levels of silicon and aluminum usually depend upon one another, such that by maintaining high levels of dissolved aluminum during an acidizing operation, silicon dissolution can be promoted as well. Silicon and aluminum dissolution in a fluid can be promoted by fluoride ion complexation, and aluminum dissolution can also be promoted by chelation. By using a chelating agent to promote aluminum dissolution, fluoride ions can remain free to coordinate silicon and promote its dissolution.
Chelation of calcium ions has not typically been effective to suppress the precipitation of calcium fluoride during conventional acidizing operations. Without being bound by any theory or mechanism, it is believed that this difficulty is due to the high formation constants of most aluminum complexes relative to the corresponding calcium complexes and the different pH ranges at which these complexes most effectively form, thereby leaving insufficient amounts of chelating agent free for calcium ion complexation. As an alternative to chelation, one approach that has been used to address the co-presence of calcium ions and siliceous materials during acidizing operations has been to carry out an initial acidizing step to solubilize and remove a substantial portion of a carbonate material, but not an appreciable portion of a siliceous material, from a subterranean formation. Thereafter, an acidizing operation can be conducted with hydrofluoric acid to solubilize the siliceous material, often after conducting one or more flushing operations to remove a substantial portion of the calcium ions and/or alkali metal ions from the subterranean formation. These types of multi-step acidizing operations can expensive, time-consuming and problematic to carry out. Moreover, it can be difficult to remove all of the calcium ions or alkali metal ions from the subterranean formation in this manner.